Hardness of metal may be determined by indenting a metal specimen at a known force using a known device and measuring either the depth or diameter of the indentation. Diametric testing is most commonly referred to as Brinell testing, and is described in ASTM E10 (10-78), incorporated herein by reference.
In Brinell testing, a ball penetrator, either 5 mm or, more typically, 10 mm in diameter, is applied to a test specimen surface with a predetermined load to produce a generally spherical indentation in the specimen surface. Knowing the applied force, the Brinell hardness of the test specimen can be determined from the diameter of the indentation.
Current Brinell testing involves measuring the diameter of the indentation along orthogonal axes using either a small optical microscope with a graded reticle or, more recently, an electro-optical instrument with a probe which indicates or responds to, respectively, the differences in light reflected from the specimen surrounding the indentation, the raised side walls adjoining the indentation and the indentation itself. Each type of instrument can be positioned over and against a specimen. Each instrument generates an image of the underlying specimen surface. The operator moves the instrument to try to center the instrument over the center of the indentation. Each instrument also typically employs light focused or otherwise directed through at least part of the instrument so that when the central axis of the scope or probe is perpendicular to the specimen surface, the light also strikes the specimen surface beneath the instrument to illuminate that area including the indentation.
A major problem with each type of apparatus is the difficulty in locating and then accurately centering the instrument with respect to the indentation. In most instances, a field of view relatively large with respect to the size of the indentation must be provided in order to permit the operator to view a sufficiently large area of the specimen surface to orient the probe with respect to the indentation. In some instances, due to a roughness of the specimen surface and/or the polish of the side walls of the indentation, the contrast between light reflected from the undisturbed surface and the indentation is relatively low, making it difficult for the operator to optically identify the indentation and the center of the indentation. While the newest Brinell hardness-measuring electro-optical probes are configured to compensate for misalignment of the probe with respect to the central axis of the indentation, such misalignment between the indentation and the central optical axis of the probe can still lead to some degradation of accuracy in the diameter measurement, particularly if the probe is significantly off center.
Because a relatively large field of view is typically provided to the optical system to permit the operator to find the indentation, the resolution of the optical system is further reduced.
Another problem arises with the electro-optical probes in connection with the miniaturization of such systems for portability. Stationary systems can provide a CRT to image the specimen surface for the operator. Existing portable systems use liquid crystal displays to depict the specimen's surface and impression for the operator. The processing capability required to generate and maintain these specimen surface reproduction displays are typically a significant portion to total processing capability of the portable unit. Consequently, such units take undesirably long periods of time to perform measurements and provide a determination of Brinell hardness.
It would be desirable to provide a means and a method to simplify alignment of a Brinell hardness optical measuring apparatus, particularly one having an electro-optical probe, with a Brinell indentation which does not adversely affect the measurement results.
It also would be desirable to provide the means and method to simplify portable Brinell hardness measuring apparatus with electro-optical probes which dispense with the need to generate a display duplicating the specimen surface for the operator in order to align the probe with the Brinell impression for accurate Brinell hardness determination.